Information storage and retrieval systems, particularly those used in computer systems, typically record data magnetically, optically or magneto-optically onto several types of storage media, rotating magnetic or optic tape or disks for example. Such storage media may be utilized for document files, computer output memories, compact disk players, hard disk drives and the like. Depending on the type of storage media and recordation system, information may be recorded only once and then retrieved many times or it may be recorded, retrieved, erased and re-recorded indefinitely. Generally, the media of optical storage systems can only be written once and read many times, such as compact disks.
The limiting feature of tape recording systems is that the media deteriorates with use and time. Although the degeneration does not occur as quickly, magnetic disk media also wear and have a limited life span. And while there are removable magnetic recording systems, typically, these systems are not suited for removability and long term reliability is a constant problem with such systems, due to the media's susceptibility to data corruption and erasure by outside stimuli, such as stray electro-magnetic fields. On the other hand, magneto-optical storage has the advantage of indefinite recording and erasure cycles without media deterioration problems and the media has the added advantage of removability and transportability between drives, as the media with its built in dust protection and non-contact operation is remarkably stable against normal outside influences.
Data stored on disks, whether magnetic, optical or magneto-optical, is contained within thousands of spiral or concentric tracks about the disk center. The total number of tracks and thus the storage capacity of the disk depends on the diameter of the disk utilized and the method of recordation of the disk. The amount of information that can be stored per unit area on the optical or magneto-optical media surface is much greater than the amount that can be stored on magnetic media, because the precision of an optical stylus is about 1 .mu.m, allowing the tracks to be spaced closely together. However, the track spacing of magnetic disks is limited to greater than 15 .mu.m, due to track runout and the signal to noise considerations of electromagnetic fields. Accordingly, the recording density of an optical or magneto-optical disk is between 10 and 100 times greater than that of a magnetic disk.
In both magnetic recording and magneto-optical recording, information is stored on a storage disk by orienting the magnetic field of the media at given points or bits along a given track. In order to record, access and read data on a disk, a transducer head in the case of magnetic recording or an optical head as shown in FIG. 1 in the case of magneto-optical recording is moved along a generally radial path across the surface of the storage disk as the storage disk is rotated. The generally radial movement of the transducer head in the case of magnetic recording or the optical head in the case of magneto-optical recording will either follow a straight line path or an arcuate path, depending upon whether a linear or rotary actuator is utilized to position the head.
The principles of magneto-optical storage are well known. Information is recorded on and erased from a thin film of magnetic material which is deposited on a substrate of suitable material. Information is encoded and stored in a sequence of magnetic bits oriented normal to the storage media surface in either of two possible orientations, north pole up or north pole down for example. To erase a track, all of its magnetized bits are oriented in one direction. Typically, for magneto-optical media, the magnetic or coercive force required to reverse a magnetic bit from, say, north pole up to north pole down, varies greatly with the temperature of the media. At room temperature, magneto-optical material is relatively resistant to changes in magnetization. The measure of this resistance is called coercivity. The coercivity of the material used in magneto-optical recording can be readily altered at a high temperature, called the Curie point. At the Curie point, about 150.degree. C., the coercive force necessary to reverse the magnetization decreases substantially and the magnetization may be reversed by a relatively small magnet or an electromagnetic device.
Typically, magneto-optical storage devices comprise an optical head including lasers, collimating lenses, beam shaping prisms, beamsplitters, plane mirrors, an objective lens, a focus positioner, a tracking positioner, collecting lenses and detectors, as noted in prior art FIG. 1. These components are, as would be expected, complex, expensive and increase the mass and size of the optical head. Referring now to prior art FIG. 1, a conventional magneto-optical head assembly 27 and its operation will now be described. During a recording operation, a laser diode 10 provides the heat source necessary for a storage media 20 to reach its Curie point. A 4 or 5 mm laser beam 29 provided by laser diode 10 passes through a grating 11, a lens 12, a polarizer 13 and a beamsplitter 14 to a movable reflecting mirror 15 and is focused at a point 28 (which represents 1 bit of information) on the magnetic material 20 of a storage disk 18 by a movable objective lens 17.
In this manner, bit 28 on the storage disk 18 can be heated, thereby lowering the coercive force required to change the magnetization of the bit. A magnetic field generated by coil 19 will cause the orientation of the magnetic domain of bit 28 being heated by the laser beam 27 to be reversed. When the laser beam is turned off, heated bit 28 is cooled in the magnetic field, thereby freezing the bit in the desired orientation. The magnetic field generated in the coil 19 by a current is in one direction for writing and in the opposite direction for erasing.
Information is read from the magneto-optical storage disk using a laser beam of reduced intensity. Thus, the magnetic film's temperature increase and corresponding coercivity decrease are small enough that the direction of magnetization is not changed. Because of the magneto-optic phenomenon known as the Kerr effect, when a linearly polarized laser beam is reflected from the vertically magnetized film 20, the plane of polarization of the laser beam will be rotated as a function of the magnetic orientation of the bit. The polarization of laser beam portions reflected from bit positions on the disk is detected by opto-electronic detector circuitry, such as photodetectors. Signals from the detecting circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
For data retrieval, the laser beam emitted from the laser diode or other suitable light source 10 is first linearly polarized by the polarizer 13. Then it passes through a beamsplitter 14 and is reflected by the mirror 15 through a lens 17 onto the disk 18. The laser beam 29 is then reflected by the magnetic film 20 of the storage disk 18. The beam 29 reflected from the film 20 of the disk 18 will have its plane of polarization rotated about 0.25.degree. to 0.5.degree. depending of the magnetic orientation of the bit due to the polar Kerr effect. Typically, magneto-optical storage films have Kerr rotation angles of less than 1.degree.. The reflected beam 29 then passes through the lens 17 and is reflected by the mirror 15 to the beamsplitter 14 where some of the beam is passed through the waveplate 21 to the polarizing beamsplitter 22, which splits the laser beam 29 into two orthogonal polarization states p=29a and s=29b, for example. Beams 29a and 29b then pass through cylindrical collecting lenses 23 and 24 and are detected by photodetectors 25 and 26, respectively. Photodetectors 25 and 26 transform the light irradiance from beams 29a and 29b into photoelectric signals i.sub.p and i.sub.s, which are then subtracted to yield a signal representative of the bit of information being read. The detected signal is then processed to extract the information contained therein. This differential type of detection is superior to other methods of signal detection as it provides some common mode noise reflection and has an improved signal to noise ratio.
Due to the many optical elements, the typical optical head measures several centimeters on a side, is 10 to 15 millimeters high, and weighs about 100 grams. Owing to its size and weight, the standard optical disk assembly is inferior to magnetic disk assemblies in its track-seek time, it's use in multi-head, multi-surface, multi-disk drives, and its use in size critical applications such as lap-top computers and notebook computers. Furthermore, optical and magneto-optical assemblies also entail a complex assembly and adjustment process, as the reduced track spacing and resulting bit densities require greater precision than magnetic storage systems. Due to the number of expensive components and corresponding complicated assembly, the average optical or magneto-optical storage assembly is much more costly than the average magnetic storage assembly.
Accordingly, there is need in the field of information storage systems for optical and magneto-optical assemblies, that have the customary advantages of stable media and large areal density, but that are also compact in size, light weight, less complex, and more economical. There is further need in the field for an improved optical storage assembly having multi-surface, multi-disk capabilities. The present invention meets these and other needs.